Transformers operate on the principle that when two wires are arranged in proximity to each other and an alternating current is passed through one of the wires, an alternating current is induced in the other wire by an effect known as electromagnetic induction. By winding the wires into coils and placing the coils along a common axis the amount of electromagnetic coupling and thus the amount of induced current will be increased over straight, parallel wires. The coupling is increased yet further by winding the two coils on top of each other. The coupling can also be increased by placing a ferromagnetic substance, referred to as a core, within the coils.
Over time cores have been improved to minimize losses. In the case of low frequency applications such as power transformers used in the national grid (typically 50-60 Hz) in order to reduce eddy currents in the core, which cause heat losses, the cores are typically implemented in layers.
In the United States electrical power intended for commercial and industrial applications is produced as three phase. For home use the power is typically also generated as three phase but in most applications only one phase is used, the other phases being used for other homes.
As mentioned above, one important issue in transformer design is energy loss, and in the energy distribution industry the opportunities for energy loss are numerous. From the generating station, electric power is transmitted at high voltages along power lines using step-up transformers, also referred to herein as generation transformers. Various stages of step-down transformers, including substation transformers and distribution transformers are then used to “step down” the voltage to usable levels, e.g. 110-240 volts, for residential and industrial users. An estimated 10% of all electricity generated is lost because of distribution inefficiency. Two types of losses can be identified in transformers: load losses or coil losses that vary depending on transformer loading, and no-load losses or core losses that occur in the magnetic cores and take place over the life of the transformer regardless of the load. No-load losses represent a significant portion of the energy lost during power distribution. It is therefore not surprising that much work has gone into improving transformer cores.
For ease of understanding the various issues involved in transformer manufacture, two types of transformers should be distinguished: single phase, and three phase transformers.
In the case of single phase transformers, a single primary winding shares its electromagnetic flux with a single secondary winding. In order to improve flux flow, ferromagnetic cores have commonly been used to provide a common flux path for the two coils. One single phase core configuration is the toroidal transformer core 100 shown in FIG. 1. However, it will be appreciated that once the core is formed, in order to get the copper windings for the first coil 102 onto the core 100, a coil bobbin that is small enough to pass through the window 106 of the core has to be passed repeatedly through the window in order to wind the coil onto the core. In the case of the second winding 106 the bobbin has to be small enough to accommodate the reduced window size caused by the first winding. An alternative approach is to form the windings separately and cut the core in order to slip the windings onto the core. The cutting of the core however causes numerous breaks in the continuity of the core material, which leads to interference in the magnetic flux path and core losses. This is typically addressed by subsequently annealing the core to minimize these losses, as is discussed in greater detail below.
In three phase power the primary and secondary windings are either connected as a delta connection (FIG. 2) or a wye connection (FIG. 3).
A variety of core configurations have been developed over the years, including the E-core, as shown in FIG. 4, which includes a three-legged section 400 in the form of an E and a bar section 402 that closes the open side of the E-section. The E includes a middle leg 404, a top leg 406, and a bottom leg 408 extending from a yoke 410.
E-cores are universally used at 50 and 60 Hz and implemented in either shell-wound configuration (primary and secondary windings wound on top of each other around the middle bar or leg 404) or core-wound configuration (the primary and secondary windings are wound around the top leg 406 and bottom leg 408, respectively). In order to reduce eddy current losses the core is typically made up of thin layers of metal stacked on top of one another. For instance in the E-core the yoke 410, legs 404, 406, 408 and bar 402 are cut to length and to the appropriate shape from a strip of metal that is typically delivered in the form of a reel. The various cut sections are then stacked on top of one another in layers to form the desired configuration. At the corners various overlapping layer configurations have been developed, e.g., butt lap and step lap configurations to minimize the losses caused by the flux direction change at the corners.
One family of transformers that has evolved to avoid some of the problems associated with core losses at the corners of the core, involves what will be referred to as wound transformer cores. Instead of stacking layers of metal on top of each other to define the leg and yoke sections of the core, the core is formed by winding several multiple-layer rings of metal and combining the rings in different configurations to define a core
Large single phase wound cores, and some three phase wound cores have been produced by Cogent Power and Metglas. The three phase cores made by Cogent Power, Inc of Burlington, Ontario include a similar design to that of Metglas, involving a 5-leg design comprising 4 rounded-square, annular or toroidal core elements arranged side by side as shown in FIG. 5. Another Cogent Power three phase arrangement makes use of three rounded-rectangular configuration cores with one large toroidal core forming the outer perimeter of the structure and two smaller toroidal cores arranged inside the larger one as shown in FIG. 6. These cores have the advantage of avoiding the overlapping core layers at the corners and thereby provide a continuous flux path prior to the coils being place (or landed) on the core. However, in order to place the copper windings or coils onto the core legs, these constructions commonly involve cutting the core material and rejoining the cut strips after the installation of the coils. The cutting and rejoining process again creates breaks in the core layers that significantly increase the core losses
Yet another configuration is discussed in U.S. Pat. No. 6,668,444 to Ngo, filed Apr. 25, 2001 and issued Dec. 30, 2003. This is shown in FIG. 7, and involves cutting a strip of core material into strip segments, which are then assembled into groups 700, which are in turn arranged in staggered configuration to define a step lapped packet 702, whereafter multiple step-lapped packets are arranged on top of one another and connected to adjacent sets of step-lapped packets. It will be appreciated that although this core construction makes use of layers having a wound configuration that avoids sharp corners with the breaks in the core layers, it nevertheless typically involves hundreds and even thousands of breaks in the amorphous metal strip causing numerous interruptions in the flux path, which leads to losses in the core.
This has led to the development of core configurations that are wound but avoid the need to cut the core legs in order to place the transformer coils on the legs. These uncut, wound cores will, for purposes of this application, be defined as continuous flux path transformer cores since they avoid sudden direction changes and breaks in the core layers. In particular, in order to achieve this continuous flux path configuration, the transformer coils need to be wound onto the legs of the core instead of being wound separately and then placed on the legs. This can be accomplished in one of two ways. One approach is by the use of a bobbin passed through the window of the transformer core. As mentioned above, the use of a bobbin is, however, very limited by design constraints since it requires enough window space between the core legs to allow the bobbin to pass through the window even when the other legs are already wound with coils that have the effect of reducing the window size.
The alternative approach, and the one that is the basis of the present invention involves the use of winding tubes that are attached around the legs in a rotatable fashion and thus allow coils to be wound onto the legs by rotating the tubes. This, however, requires leg cross-sections that are substantially round in order to minimize the air gap between the core legs and the coil windings. For purposes of this application, the term substantially round cross-section will refer to a multi-sided cross-section that has more than 4 sides (more sides than a simple square or rectangle) to increase the fill factor of core material within the circle defined by the coil windings that are wound around the core leg and thus provide a higher fill factor than that provided by a square or rectangular cross-sectional core leg. In order to achieve such a non-rectangular cross section, the cores are built up of a complex set of beveled rings, which involves a process that is significantly more complex and requires more manipulation than is the case with a simple set of toroidal core elements.
One such core configuration, is the hexaformer core, which has been publicly available since Mar. 16, 2000 and is shown in FIG. 8. This core configuration is described in greater detail in U.S. patent application Ser. No. 09/623,285 (U.S. Pat. No. 6,683,524 to Hoglund, filed as a PCT application on Sep. 2, 1999). The hexaformer core defines legs with a hexagonal cross-section, which is sufficiently round to permit winding coils on the legs using winding tubes while maintaining a high fill factor (core material in the circle defined by the coils that are wound on the winding tube). Another continuous flux path core configurations that permits winding on the leg is the Wiegand configuration as described in U.S. Pat. No. 2,544,871 to Wiegand, filed Apr. 24, 1947 and issued Mar. 13, 1951, which makes use of parallel sided strips of wound material. Two other continuous flux path cores that allow winding on the leg are the Haihong core produced by Haihong in China, and the Manderson core described in U.S. Pat. No. 4,557,039 to Manderson, filed Jul. 20, 1982 and issued Dec. 10, 1985, which differ from the hexaformer core and the Wiegand core in that they make use of wound material with tapered sides.
In an alternative approach, in order to reduce core losses, the use of an amorphous metal alloy as a core material has found a significant amount of interest. However, due to the nature of amorphous metal, which will be discussed in greater detail below, cores made from amorphous metal have maintained a simple configuration in which the legs of the core have a simple square or rectangular cross section. One such amorphous metal transformer is described in U.S. Pat. No. 6,844,799 to Attarian which describes the use of amorphous metal laminations. One type of amorphous metal described in Attarian is the use of a cobalt-based (Co-based) amorphous metallic alloy, or a cobalt-iron (CoFe) alloy that may also include vanadium (e.g., CoFe—V having 49% Co, 49% Fe, and 2% Vanadium (V)). As described in the Attarian patent, amorphous metallic alloys are produced by rapid solidification of molten metal and exhibit excellent magnetic properties as described in the article entitled “Amorphous Metallic Alloys” in the undated publication entitled “AMOS® Amorphous Cores” by AMOTECH (Advanced Material On TECHnology).
However, as mentioned above, amorphous metal has physical characteristics that make it much more difficult to work with than silicon steel. Amorphous is by its nature a very thin, slippery material that lacks rigidity and therefore is extremely floppy and difficult to handle. The layers of amorphous metal used to build up a core are typically to significantly thinner than silicon steel layers used in silicon steel transformers. Amorphous metal layers have a thickness of the order of only 0.001 inch (0.0254 mm) since the production of amorphous metal requires the amorphous alloy to be cooled quickly in order to avoid grain structure from forming and thus requires the alloy to be manufactured very thinly.
Accordingly, the layers are of the order of 8 to 12 times thinner than any silicon steel conventionally used in transformers, and are very slippery. Even when built up as hundreds of layers of amorphous material, it remains floppy and has none of the self-supporting rigidity found in silicon steel built up to a similar thickness. As a result amorphous metal transformer cores have in the past been largely limited to single phase transformers with a simple C-shaped core or annular core, which is typically shaped like a square doughnut with rounded corners such as those described on the Metglas Website www.metglas.com, or as crude three phase cores involving multiple annular cores arranged side by side to define a 5-legged or 3-legged design. These cores have legs with a square or rectangular cross-section, and are thus not suitable for winding transformer coils onto the legs by means of coil tubes since the fill factor between the coils and the core would be too low.
The Ngo core design described above has also been implemented using amorphous metal but again the leg cross section is a simple square or rectangle and is therefore not suitable for winding transformer coils on the legs by means of winding tubes.
In order to avoid the losses and limit the need for annealing typically involved with cut cores, the present invention provides for a continuous flux path three phase core implemented at least in part from amorphous metal.